This invention relates to a snubber circuit for a switching power source such as a switching regulator and, more particularly, to a snubber circuit of this type with an improved efficiency.
In a switching power source such as a switching regulator, energy accumulated in a coil of a transformer while a switching element is on causes resonance with ambient floating capacity when the switching element is turned off with resulting occurrence of ringing with an accompanying overshoot. Ringing waves thus caused impart considerable stress to the switching element as well as produce undesirable noise to the outside. For restraining this ringing and thereby preventing occurrence of disorder in the switching element and reducing noise, a snubber circuit is connected to the switching element.
A switching power source including a prior art snubber circuit is shown in FIG. 2. A dc power source 10 is connected to a primary coil 16 of a transformer 14 through a switching element 12 and an output is produced from a secondary coil 18. A snubber circuit 26 is constructed of a series circuit of a diode 20 and a capacitor 22 which is connected in parallel with the primary coil 16 and a resistor 24 which is connected to the terminals of the capacitor 22. When the switching element 12 is turned off, energy which has been accumulated in the primary coil 16 of the transformer 14 is transferred to the capacitor 22 and consumed by the resistor 24 whereby the amplitude of ringing waves is held at a relatively low level.
In the prior art snubber circuit shown in FIG. 2, voltage which is substantially equivalent to power voltage E of the dc power source 10 (e.g., in the case of an AC/DC converter, DC 140 v in an area where AC 100 V is used and DC 320 V in an area where AC 230 V is used) is constantly applied to the terminals of the capacitor 22 and, therefore, current flows continuously through the resistor 24 (about 10 kohm to 20 kohm). This causes a loss in the order of 1 W to 3 W for obtaining the benefit of the snubber circuit 26 and thereby reduces efficiency of the snubber circuit 26 to that extent. Moreover, the prior art snubber circuit is disadvantageous in that it requires space for providing the resistor.
It is, therefore, an object of the present invention to provide a novel snubber circuit which has eliminated the above problem of the prior art snubber circuit and is capable of reducing a loss caused in the circuit.